With the progress of science and technology, the popularity of mobile audio equipment has been increased. As a mobile audio output device, earphones play an important role in people's life.
However, when most of earphones are worn by a user, the housing protrudes from a cavum conchae of the user and is exposed to the outside, such that the earphone tends to slip out from the user's ear. To address the problem of slipping, earphones with smaller size which can be placed in the ear of the user are employed. However, the design size of the conventional earphone usually does not completely fit the size of cavum conchae. If the size of earphone is too big, the concha will be abutted and the user's ear is compressed; if the size of earphone is too small, the earphone is easily to be slipped out from the user's ear. Therefore, it is inconvenient.